Process for producing stable silica sols



Patented Dec. 4, 1951 PROCESS FOR PROlggICSING STABLE SILICA Joseph M. Rule, Cleveland, Ohio, assignor to E. I. -du Pont de Nemours & Company, Wilmington, DeL, a corporation of Delaware No Drawing. Application September 8, 1950, Serial No. 183,901

13 Claims.

This invention relates to processes for producing improved silica sols, and is more particularly directed to a process comprising building up the size of particles in a substantially saltfree silica sol of substantially spherical, unaggregated, dense particles by adding to such a sol a substantially salt-free sol of active silica and. heating the combined sols at above 60 C. while maintaining the pH in the range from 7.5 to 10.7, the molecular ratio of SiOz to base, expressed as M20, in the product being from 130:1 to 500:1.

In application Ser. No. 65,536, filed December 15, 1948, Bechtold and Snyder which issued as Patent No. 2,574,902 on Nov. 13, 1951 have described processes in which silica particles in a heel of previously heated silica sol are built up in size by adding further quantities of sol containing low-molecular-weight silica and heating the combined sols. They disclose that the buildup process should be carried out at a silicazalkali oxide mole ratio of from 60:1 to 130.1, and that sols may be produced having silica concentrations up to 35 In my copending application Ser. No. 183,902, filed concurrently herewith, I have described processes in which an alkali metal hydroxide is added to a silica sol containing built-up silica particles such as those prepared by the abovedescribed build-up method of Bechtold and Snyder, the sol'before adding the hydroxide being substantially free of soluble salts and alkalis and the amount of hydroxide added being sufficient to give the sol a silicazalkali metal oxide mole ratio of from 130:1 to 500:1, and I have claimed therein certain sol products which can also be made by processes of the present application. One of the ways described in the copending application Ser. No. 183,902, for rendering the starting sol sufiiciently free of salts is to pass the sol successively through columns of cation and anion exchange resins, but it will be noted that such ion-exchange is effected on sols of build-up particles rather than'on sols containing active silica.

When a silica sol is made from commercial sodium silicate solution by first removing the sodium by ion exchange, then adjusting the SiO2:NazO ratio to from 60:1 to 130:1, then effecting build-up of the silica particles, then deionizing the built-up sol, and finally adjusting the ratio to the desired figure by adding back alkali, it will be evident that considerable handling of the silica sols is necessary, with the usual yield losses, and that the ion-exchange facilities must be substantial. It is further evident that processes which minimize both the amount of handling of the sols and the investment in ion-exchange facilities required will ofier definite economic advantages.

It is, therefore, an object of this invention to provide novel processes for making silica sols. Another object is to provide processes whereby silica sols of high purity and stability can be made from commercial grades of sodium silicate. Another object is to provide such processes in which unwanted cations and anions are removed from the sols by ion-exchange processes. Another object is to provide processes for producing silica sols Which can be concentrated to high silica contents withoutgelling. Another object is to provide processes for producing silica sols which are stable and have SiOzzMzO ratios in the range of 130:1 to 500:1. Further objects will appear hereinafter.

The foregoing and related objects are accomplished according to the invention by processes comprising building up the size of particles in a substantially salt-free silica sol of substantially spherical, unaggregated, dense particles by adding to such a sol a substantially salt-free sol of active silica and heating thecombined sols at above C. while maintaining the pH in the range from 7.5 to 10.7, the molecular ratio of S102 to base, expressed as M20, in the product being from :1 to 500:1.

By the operation of a process of this invention as just described there may be produced silica sols which are stable even when concentrated to upwards of 35% S102, at SiO2ZM2O ratios of 130:1 to 500:1, and the processes including such a concentration step constitute a preferred embodiment of the invention. The sols produced may have a silica content of, say, 40 or 45% by weight at these ratios. Such sols may advantageously be prepared according to this invention from cheap, readily available grades of commercial sodium silicate solution. The processes, in a preferred aspect, employ ion-exchange methods for removing unwanted ions to makethe saltfree, active-silica sols, and may do so with a minimum of handling of the sols and of plant investment.

PREPARING SALT FREE SOL 0F ACTIVE SILICA The initial step in a process of this invention is to prepare an active silica sol which is substantially free of soluble salts. To do this it is first necessary. of course, to know how to measure the activity of silica. and how to determine the salt content of a sol.

Active silica is silica in a form in which it may readily undergo polymerization. For instance, when sodium silicate is acidified, active silica is released. This-active silica initially may be in the form of orthosilicic acid, Si'(OH)4, but a polymerization reaction occurs, whereby water splits out between adjacent silanol groups, with the formation of asiloxane bond. This polymerization may proceed until: a predominate proportion of the silanol groups present have been used up to form siloxane-bonds; the polymer thus formed will have a high molecularweig-ht and the-silica in it willbe :predorninately inactive. It will be understood, then, that by the term active silica is meant silica'of -low molecular weight in which. polymerization by condensation between silanol groups has not 'proceeded'to a predominate extent.

Active silica is readily'identified because a sol containng silica in the active form: when addedto-any sol' of colloidal silica under: the conditions of pH and temperature described more fully hereinafter builds up or increasesthe size ofeXisting nuclei;

Active silica is also referred ltoin various Ways-as for example inMonsantos British Patent #19,?42, datedJuly 27, 1949, it is referred to-as ionic silica, which is possibly a misnomer as to thetrue stateof the colloidalsilica in the pH range 8' to 9 and under the conditions at which particle growth occurs.

It should be understood that-monomeric silica is-not'the-only active form. I havefound. that certain polymeric forms of silica which are capableof depolymerization. under suitable conditionsare active for the-purposesof the present invention. The rate-of depolymerization under controlled conditions provides a measure of whether or not aparticular silica is active within the meaning of the term as used in describing thisinvention.

Active silica may be defined as any silica, in molecular or colloidal aqueous-solution, in such a state of polymerization that whendilutedwith sodium hydroxide solution to a pH of. 12.0 and an SiOz'concentration of about-0.02% by weightaat 30 0., in theabsence of cations other than the sodium, the silica willdepolymerize substantially completely, that is, at least 90%, to monomerin not'more than about 100 minutes.

The-amount of monomer in aparticular solution may be determined by measuring the amount of color developed with molybdic acid. It was shownby-I-Iarman (J. Phys. Chem-., vol. 31, 616-25 (1927)), that yellow silico-molybdiocomplex. acidis formed by. reaction of 'molybdic acid with crystalloidal silica but not Withcolloidal silica. I confirmed this observation, and concluded thationly monosilicic acid: forms-the yellow complex with molybdic acid. Hence,- the rate of depolymerization of silica in a dilute test solution can be foilowed over a period of time by taking samples and determining the amount of monomer. present.

Specifically, the depolymerization test is carried outby adding the silica, sampleto such quantity ofadilute solution of sodium hydroxide that the pH of the mixture is 12 (corresponding to 0.01 normality of alkali) and the total concentration of SiOz is 0.02%. This mixture is held at 30 C. for a measured time after preparation, and then tested for monomer. Five ml. of the depolymerizing solution aremixed with e5ml. of a molybdic acidireagent made up by diluting-25 mi. of M sulfuric acid and 10 ml. of 10% (by weight) am- .monium molybdate solution to a total of 225 ml.

The color developed in the mixture at 30 C. is followed witha Beckman Spectrophotometer, and reaches a practically constant value in two min utes. 'I-hev amount of monomeric silica corresponding to this :color is determined by standardizing the color. against a known amount of sodium orthosilicate. The wavelength of the light used was 400 millimicrons.

'While silica which depolymerizes completely in not more than minutes by the foregoing test may be considered to be active, it is preferred to use silica which will depolymerize completely in not more than 50 minutes, and especially preferred to use a material on which the depolymerization time is not more than 30 minutes.

The limitation that the active silica sol must be substantially. salt-free means that the con.- tent of soluble cations and anions must be very low-. The salt contentmay, of course, be determined'by chemical analysis, but a moreirapid and feasible-method of establishing that the sol is relatively free of soluble salts is to determine its specific conductance, a low specific conduct ance indicating the absenceof current-carrying ions. Ihavefound that when-the specific conductancejof, a sol, as measured at 2% SiO2 and'28 0., is not more than l'7 l0 mho/cm., the sol is satisfactory for use in a process of this invention, and I consider such a sol to besubstantially salt-free. It must be remembered that in an active silica sol the low-molecular-weight silicic aciditself contributes some conductivity, so that the actual salt content is even less than would be indicated by the specific conductance limit'of 1'7 X 10 mho/cm.

The specific conductance of a silica sol to be used. in a process of this invention may be determined-inaccordance with conventional practices, such as .thatdescribed by Glasstone, Text- Book' of PhysicalChemistry, at page 874 et seq. The specific conductance is measured at 28 C. on asolcontaining 2 percent by weight of silica expressed'as S102. More concentrated sols may be diluted withdistilled water for measurement.

The silica sol-used asthe starting material in this,v invention may be prepared by any method capable of giving a sol which is sufiiciently saltfree and in which thesilica is sufficiently active tocome within-the limits-set forth above. Methods forliberating silica are numerous and well known, and with precautions to keep temperature down, the pH low, and reaction time short, silica canrbe produced'inactive form. Methyl silicate may be hydrolyzed with water, as taught by Brimaux, Compt. rend. 98, and 1434 (1834); silicon tetrachloride may be hydrolyzed with water seewilliams. U. S. Patent 1,539,342 Silicon sulfide-may be hydrolyzed with water as Batent-2,2.44,325.

While the methods described above give sols of active silica, they will not, necessarily, be saltfree. The anion content of the sol will be considerable for instance in a process in which silicon tetrachloride is hydrolyzed, and it will be necessary to remove such anions because otherwise the salt content will become substantial as alkali is added to adjust the pH or M20 ratio to the value desired in the subsequent steps. Such methods as dialysis, or contacting the sol with anion exchange resins, may be employed for reducing the anion content.

The method of the Bird Patent 2,244,325, above-mentioned, is particularly preferred as a way of making a sol of active silica. If the sodium silicate, from which the sodium is removed in the process, is chemically pure to start with, and if no contamination is permitted to occur in the process, the resulting sol will be and remain substantially salt-free. However, for practical operation it is most feasible, and preferred, to follow the cation-exchange process of Bird with an anion exchange process, to give a sol substantially free of both cations and anions.

In the cation-removal step, any insoluble cation-exchanger may be used, the resins of sulfonated carbonaceous exchangers, or of sulfonated or sulfited insoluble phenol-formaldehyde resins, or acid-treated humic material, or other similar exchangers, being typical. Sulfonated coal, lignin; peat, or other insoluble sulfonated humic organic material may be used. Even more preferable are the insoluble resins made from phenols, such as those made from phenol itself, diphenylol sulfone, catechol, or naturally occurring phenols, as found, for example, in quebracho, and an aldehyde, particularly formaldehyde, which are modified by the introduction of sulfonic groups either in the ring or on methylene groups. Cation-exchangers which are stable in their hydrogen forms are available commercially under such trade names as Amberlite, Ionex, Zeokarb, Nalcite, Ionac, etc.

The exchanger should be initially in the acid form. It will be understood that to regenerate a spent exchanger to the acid form even moderately weak acids will often be sufficient particularly if the acidity is derived from carboxylic acids or even phenolic groups.

The exchanger is generally prepared in a granular form which is readily leached free of soluble acids or salts. If the exchanger is exhausted by use it may readily be converted to the acid form by washing with a solution of an acid such as hydrochloric, sulfuric, formic, sulfamic, or the like.

One of the preferred cation-exchange resins for use according to the present invention is an aromatic hydrocarbon polymer containing nuclear sulfonic acid groups which is designated Dowex 50 and of the general type described in DAlelio 2,366,007 and which is fully described as to its characteristics, properties, and general mode of use in the Journal of the American Chemical Society for November, 1947, volume 69, No. 11, beginning at page 2830.

For convenience of reference, suitable silica sols from which cations have been substantially completely removed by ion exchange as just de-- scribed may be referred to as half-cycle deionized sols.

Sols prepared according to the disclosure of the Bird patent above mentioned may contain varying amounts of anions such as sulfate, chloride, carbonate and bicarbonate, or the like. By carefully controlling the processes to avoid contamination by anions, the anion concentration, and hence, the specific conductance, may be held low enough so that the sol may be treated according to a process of the present invention without further adjustment of the anion content. Usin'g deionized water for making up all solutions and carefully avoiding acid contamination of the sol from the regeneration of the cation exchange resin are expedients which may be followed to keep down the anion content. However, in some circumstances it is not practicable to observe these precautions and it is accordingly preferred to reduce the anion content, if required, by passing the sol through an anion-exchanger in the basic form.

Anion exchangers are generally well known, and the composition of the anion exchangers and their mode of use are fully described in the literature. Suitable materials are mentioned, for instance, in U. S. Patents 2,438,230 and 2,422,054. A description of both cationand anion-exchangers will be found in the May, 1945, issue of Chemical Industries, in an article entitled Ion-exchange, by Sidney Sussman and Albert B. Mindler, at pages 789 et seq.

While any of the anion-exchangers described will be found satisfactory, the insoluble resins obtained b the reaction of formaldehyde with an aromatic amine are particularly useful. Such products are described, for instance, in the U. S. Patent 2,151,883 of Adams and Holmes. Reference is made, for instance, to the metaphenylenediamineformaldehyde type of anion-exchanger in the Ryznar Patent 2,438,230 stated above. A guanldine-type anion-exchanger may also be used.

As with the cation-exchangers, the technique of use is generally well understoodand the anionexchangers may be used in the manners customary in the art. It will be understood again that a considerable excess of anion-exchanger will ordinarily be used and the sulfate and chloride content will be lowered to an exceedingly low value.

By the treatment with the anion-exchanger the content of anions other than OH will drop to an exceedingly low figure and will not total more than about two hundredths to three hundredths of a per cent.

A silica sol which has been subjected to both cationand anion-exchange as above described may be referred to as a full-cycle deionized sol.

The manner of using ion-exchange resins for making a salt-free, active silica sol may be considerably varied. The resin may be stirred into the sodium silicate or silica solution and filtered off after the ion-exchange reaction has occurred. A preferred method is to place the resin in a vertical column and percolate the ion-containing solution through it. The anionand cation-exchange resins may be present together as a mixed bed, but for deionizing sodium silicate it is preferred that the cation-exchange precede the anion-exchange. In a column, the flow of solution may be downward, but in a particularly preferred process the flow is upward, since this somewhat extends the bed and permits the preparation of somewhat more concentrated efiluents.

OUTLINE OF THE PROCESS Processes of the invention are characterized by the building up of the particles in a substan the active silica in situ in the sol.

worktype of structures, and are unagglomerated,

dense particles which are not joined together into anisotropic masses. -fe,cte d as herein set out inmore detail in a variety This build-up canbe efof-ways, using active silica which is substantially'salt-free.

For-instance, a salt-free active silica can be ,heated under conditions set outbelow, and then iurther-quantitiesof salt-free active silica added untilasol of the, desired character is made.

The sols thus prepared can be concentrated by evaporation.

The addition of active silica to a sol of unaggregated particles can be carried outinavariety of ways. For example, in addition to the preferred manner above described, one could prepare This is similar to the practice of the British patent application #191742.

Silica sols of the invention preferably have an SiOzZMzO mol ratio of 130:1 to 500:1, and can be prepared with any of a variety of bases under conditions as generally described.

The mol ratio, siozuxno, herein referred to, is the'number of mols of silica present, calculated .as, SiOz, per mol of alkali metal oxide or its equivalent. In this expression, M20 is used in accordance with common practice to represent an oxide of a monovalent cation such as lithium, sodium, potassium, rubidium, or caesium, or the the theoretical ammonium and substituted ammonium ions. When M20 is the oxide of a theoretical substituted ammonium ion, the base may be a quaternary ammonium hydroxide, such as tetramethylammonium hydroxide, or an amine such as trimethylamine, the base being soluble and sufiiciently strong to have .a pKa value greater than 8. The amines may contain substitution groups, as is the case with mono-, diand tri-ethanolamines.

The oxide of a monovalent cation may be added in the form of the hydroxide or as a'soluble silicate of the cation.

PREPARATION OF HEEL SOL to the desired pH, the sol is then heated above 60 C. to form the heel. The temperature of 60 C. is a minimum, andordinarily it is preferred to heat to a considerably higher temperature, such as the atmospheric boiling temperature of 100 C. The heating is for the purpcseof converting the low-molecular-weight silica to nuclei upon which build-up may occur in the subsequent steps of the process. The time of heating required is inversely proportional to the temperature, longer times being required at 60 C. than at 100 C. Ordinarily, heating at 100 C. for a few -minutes is sufficient, and if the time required for reaching the elevated temperature is substantial,

perature is adequate.

much, or even more.

58 THE BUILD-UP After the heel has'been prepared'as above described, the build-up of silica upon the nuclei in the heel may be started. Build-up, is effected by addingfresh, unheated, salt-free active silica Solto the heel sol while maintaining the pH in the range'from 7.5 to 10.7, as measured on samples cooled to 30 0., and heating the combined sols above 60 C.

.Several ways of maintaining the pH in the specified range are available, and each may be preferred, dependingupon the circumstances. If, for instance, it is desired to produce a final product with an SiO2IM2O mol ratio in the range .from 130 to 300:1, the feedof fresh, active silica being the same during build-up and in theheel.

Alternatively, if a higher ratio product, say 400 or 500:1,ratio, is to be produced, all or most of the alkali maybe added to the heel, and the feed may'then be added to efifect build-up. In another embodiment, feed sol and alkali maybe added simultaneously, but separately, torthe heel, the rate of adding the alkali being so controlled as to maintain the pH at a predetermined value. This value will, of course, fall within the range of 7.5 to 10.7.

It must'be-remembered that the pH which will result from adding agiven amount of alkali will depend to some extent on the size towhich the silica particles in the sol have grown. Thus, alkali is more effective-in raisingthe pHas the particle size increases. Conversely, the ratio cannot be ascertained from the pH unless the particlesize is considered.

The amount of silica added to the-heel as feed should be at least equal to that originally present, and in a preferred process will be 3'to 15 times as As build-up continues, of course, the average size of particles grows, and for sols to be concentrated above .0% SiOz, the

size of particles should be grown intothe range of '13 to 60 millimicrons in diameter, while for sols to be concentrated to the size of particles may advantageously be in the range of 15 to 30 millimicrons.

'tinuously.

The temperature to which the combined sols should be heated must be at least C., and preferably should be higher. Excellent results have been obtained byheating to the boiling temperature at atmospheric pressure.

The heating may be carriedout under pressure or reflux, but since concentration of built-upsol is usually desiredanyway, it is preferredto allow free evaporation of water duringthe. heating, and tomaintain a constant volume by adding feed sol at the rate at which Water is evaporated off.

In the constant volume evaporation process just described, the rate of adding active silica sol to the heel is determined by the rate at which water can be evaporated ofi, and in a commercial plant a periodof ten or fifteen hours may be employed, but the time should not be unduly prolonged say, to several days, nor should it be so short-as 7-5 tonot-permit theadded silica to-build up on the nuclei in the heel. Adding the feed over a period of an hour or two is usually not too fast.

THE SOL PRODUCTS The built-up sols may, if desired, be concentrated to high silica contents, simply by boiling 01f water. They may, for instance, be concen .trated above 35% S102 and may even be concentrated to as high as 40 and 45% SiOz. The constant volume build-up above-described is particularly preferred in giving these concentrated sols. The concentration may advantageously be effected by boiling the sol under rapid agitation, or under a layer of oil, or under a steam-saturated blanket, or under any other condition which prevents dehydration of the-silica at the gas-liquid interface.

The sol products have SiOzZMzO mol ratios in the range from 130:1 to 500:1. They are stable against gelation at this ratio range, even at high concentrations of silica. They enjoy a substantially complete freedom from electrolytes, and contain amorphous particles which are dense, spherical, and 10 to 130 millimicrons in diameter, as described in my copending application Ser. No. 183,902, filed concurrently herewith.

EXAMPLES The invention will be better understood by reference to the following illustrative examples:

Example 1 A sodium silicate solution (2.5% S102, wt. ratio SiOz/Naz=3.25), which had been prepared by diluting a commercial sodium silicate solution (Grasselli F grade) with water, was deionized with Nalcite HCR resin and Amberlite IR-4B resin according to the following procedure:

The dilute sodium silicate solution was allowed to percolate through a column (2.25 x 30 inches) of the hydrogen form of the cation exchange resin Nalcite HCR and the efiiuent from this column was noted to possess a pH of 2.65 and to contain appreciable amounts of sulfate and chloride ions. The impure silicic acid solution thus formed was then allowed to percolate through a column (2.25 X 30 inches) of the basic form of the anion exchange resin Amberlite IR-4B in order to remove the strong acids such as hydrochloric and sulfuric. The resulting solution of low molecular weight silicic acid had a pH of 3.8, specific conductance=1.2 mho/cm., and a silica content of 2.3%. The decreased concentration of silica in the efiiuent appeared to be the result of dilution by the water in the ion exchange columns and not the result of an adsorption of silicic acid by the ion exchange resins.

The low molecular weight silicic acid solution was then converted to a stable silica sol by adjusting a portion (1500 ml.) of the deionized silicic acid to an SiOz/NazO ratio of 150 with 1 N sodium hydroxide and then boiling under reflux at atmospheric pressure for 30 minutes. The pH increased from an initial value of 7.85 to a final value of 8.75 during the refluxing of the solution.

The material was then transferred to an appro- 2.25 centistokes. The remainder of the build-up was performed over a 17% hour period by the addition of 19 liters of deionized silicic acid and the appropriate amount of sodium hydroxide to maintain the SiOz/NazO ratio of 150. The final sol had a pH of 9.60 and contained 44% SiOz. The viscosity of this concentrated sol was 684 centistokes but upon dilution to a silica content of 10% and adjusting the pH to a value of 10, the relative viscosity measured as prescribed was 1.2. The concentrated sol was slightly opalescent in appearance but was relatively clear as compared to most common silica sols. The material did not gel upon 1 month of heated storage C.), and the specific conductance, as measured at 28 C. and a silica content of 10%, was 4.8)(10- mho/cm.

This concentrated silica sol was fairly compatible with most water miscible organic solvents and could be incorporated into many polymers such as polyvinyl alcohol, natural and synthetic rubber latices, gelatin, and urea-formaldehyde polymers to yield many useful materials.

Example 2 The low molecular weight silicic acid solution (SiO2=2.3%) prepared in Example 1 couldalso be converted to a stable silica sol according to the following procedure:

A heel was prepared from the low molecular weight silicic acid solution (500 grams) by re fluxing it with sodium hydroxide (4 ml. of 1 N aqueous solution) for a period of 30 minutes. Then an additional quantity (8500 grams) of the low molecular weight silicic acid solution and sodium hydroxide (31 ml. of 1 N aqueous solution) were added during a period of 13 hours ac cording to the constant volume build-up procedure described above. The resulting s01 of builtup particles had a pH of 8.9 and an SiO2/NazO ratio of 200.

The final sol did not gel during 1 month of heated storage (95 C.), and the specific conductance as measured at 28 C. and a silica content of 10% was 4.2 10- mho/crn. An electron micrograph of the sol particles showed the silica to be in the form of discrete spherical particles with an average diameter of about 20 millimicrons.

I claim:

1. In a process for producing stable silica sols, the steps comprising preparing a substantially salt-free sol of active silica, the sol having a specific conductance, as measured at 2% S102 and 28 0., of not more than 1'7 X 10 mho/cm., adding a suitable amount of a base having a monovalent cation to adjust the pH to between 6 and 9, heating the adjusted sol above 60 C. to form a heel, adding to the heel an additional quantity of a substantially salt-free sol of active silica while maintaining the pH in the range from 7.5 to 10.7, the amount of silica added being at least equal to that originally present, and heating the combined sols above 60 0., whereby build-up of the added silica upon that present in the heel is effected to give a sol product stable against gelation, total alkali additions being adjusted so that the product has an SiOzZMzO ratio of :1 to

asvmsa 500:1, and concentrating the silica solobtained cific conductance, as measured at 2% SlOz and 28 C., of not more than 17 l0 mho/cm., adding a suitable amount of abase having a monovalent cation to adjust the pH to between 6 and 9, heating the adjusted sol above 60 C. to form a heeL adding to theheelan additional quantity of a substantially salt-free .sol of active silica while maintaining the pH in the range from 7.5 to 10.7, the amount of silica added being at least equal .to that originally present, and heating the combined sols above'60 C., whereby build-up of the added silica upon that present in the heel is effected, and concentrating the sol obtained to a silica concentrationof from 35 to 45% by weight of S102, total alkali additions being adjusted so that the product has an SiOzzMzO ratio of 130:1 to 500:1.

3. In a process for producing stable silica sols, the steps comprising preparing a substantially salt-free sol of active silica, the sol having a specific conductance, as measured at 2% S102 and28 C., of notmore than -17 '10- mho/cm., adding a suitable amount of .sodium hydroxide to adjust the pH to between 6 and 9, heating the adjusted sol above 60 C. to:form a heel, adding to the heel an additional quantity of a substantially salt-free sol of active silica while maintaining the-pH in the range'from 7.5 to "10.7, the amount of silica added being at least equal to that originally present, and heating the combined sols above 60 0., whereby build-up of the added silica upon "that present in the heel is efiected to give a sol product stable against gelation, total alkali additions being adjusted so that the product has an 'SiOz:M2O ratio of 130:1 to 500:1, and concentrating the silica solobtained to a silica-concentration'of not more than 45% by weight of SiOz.

4. In a process for producing stable silica sols, the steps comprising preparing a substantially salt-free sol of active silica, the sol having aspecific conductance, as measured at 2% SiOzand 28 C., of not more than '17 10 rriho/cm., adding a suitable amount of lithium hydroxide to adjust the, pH toflbetween 6 and 9, heating the adjusted-sol above 60 C. to form "a heel, adding to the heel an additional quantity of a substantially salt-free sol of active silica while maintaining the pH in the range from 7.5 to 10.7, the amount of silica added being at least'e'qual to that originally present, and heating "the combined sols above 60 C., whereby build-up of the added silica upon that present in the heel is ef-- fected to give a sol product stable against gelation, total alkali additions being adjusted so that the product has an SiO22M2O ratio of 13021 to 500:1, and concentrating the silica sol obtained to a silica concentration of not more than 45% by weight of SiOz.

5. In a process for producing stable silica sols, the steps comprising preparing a substantially salt-free "sol of active silica, the sol having a specific conductance, as measured at "2% SiOz and 28 C., of not more than l7 10 'mho/'cm., by effecting contact, successively, between a sodium silicate solution and a cation-exchanger in the hydrogen form and an anion-exchanger in-the basic form, adding a suitable amount of a base 'having 'a'monovalent cation 'to adjust the 12 pH ito' between-:6 and 9, heating the adjustedsol above 60 C. to'form a "heel, addingto .the heel an additional quantity of a substantially saltfree sol of active silica while maintaining the pH in the range'from 7.5 to 10.7,theamountof silica added being :at least equalxtoithat originally present, and heating the combined sols above 60 C., whereby build-up of the added.

silica upon that present in the heel is effected to give a $01 product stable against gelation, total alkali additions being'adjus'ted so that the product has an'sioztMzo ratio of :1 to 500:1, and concentrating the silica sol obtained to a silica concentration of not more than 45% by weight of $102.

6. "In 'aprocess for'producing: stable silica sols, the steps comprising preparing a substantially salt-free :sol of active silica, the sol having a specific conductance, as -measured .at 2% SiOz and28 C.,-.otnot more than 1-7 10- -mho/cm., adding .a suitable amount of a base having a monovalent cation to adjust the ,pH to between Gland 9,.heating-theadjusted sol above 60 C. to form--a-heel, adding :to the heel an. additional quantity of a substantially salt-.free sol of active silica while maintaining the vp'I-I in the range from 7.5 to 10.7 and the temperature at the boiling point of the sol, whereby water is evaporated, the additional quantity of sol of active 'silicaibeing added at .sucha rate,relative to'the rate of evaporation, that thevolume of the combined sols remains substantially constant, and the amount of silica added being at least equal to that originally present, whereby build-up of the added silica upon that present in the heel is effected to give a sol product stable against gelation, total alkali additions being adjusted so that the product has-an SiOzZMzO ratio of 130:1 to 500:1, and concentrating the silicasol obtained to -'a silica concentration of not more than=45% by-weight of SiOz.

7. In a process for producing stable silica sols, the steps comprising preparing a substantially salt-free sol of active silica, the sol having a specifivconductance, as measured-at 2% 55.02 and 28 C., of 'not more than 1 7 '10 mho/cm., byzefiecting contact, :successively, between a :sodium silicatersolution and a cation-exchanger in the hydrogen form and an anion-exchanger in the basic .form, adding a suitable amount'iof 'a base having a monovalent cation .to adjust the pI-Lto between 6 and' 9, heatingtheadjusted sol above-- 60 1G. to form x a heel, adding to the :heel an additional quantity of a substantially saltfreesol of':active silica 'While -main-taining' the pH in-vthe range1from 7.5 to 10.7 and the temperature'at the boiling :;point of ithesol, whereby water is evaporated, the-additional quantity -:of sol of active-silica; being iaddedat such arate, relative-totherate ofevaporation, that the volume of the combinedsols remains substantiallyconstant, .and the amount of silica added "being: at least equal to: that originally :present, whereby build-up of the added silica upon :that present in the heel'is effected to give a-solrproduct Stable against gelationptotal alkali additions being adj usted-so'that-theproduct has an S102 M20 ratio of 130:1 'to 500:1, and concentrating the silica sol-obtained to-a silica concentration of not more than 45 'by weight of SiOz.

8. In a process for producing stable silicasols. the steps comprising preparing a substantially salt-freesolof active silica the sol having a-spevcific conductanca'as measured at 2% SiOz and 28-C.,'of not more than 17x10- mho/cm., by

effectingc'ontact, successively, between a sodium silicate solution. and a cation-exchanger in the hydrogen form and an anion-exchanger in the basic form, adding a suitable amount of sodium hydroxide to adjust the pH to between 6 and 9, heating the adjusted sol above 60 C. to form a heel, adding to the heel an additional quantity of a substantially salt-free sol of active silica while maintaining the pH in the range from 7.5 to 10.7 and the temperature at the boiling point of the sol, whereby water is evaporated, the additional quantity of sol of active silica being added at such a rate, relative to the rate of evaporation, that the volume of the combined sols remains substantially constant, and the amount of silica added-being at least equal to that originally present, whereby build-up of the added silica upon that present in the heel is efiected to give a sol product stable against gelation, total alkali additions being adjusted so that the product has an SiOzZMzO ratio of 130:1 to 500:1, and concentrating the silica sol obtained to a silica concentration of not more than 45% by weight of S102.

9. In a process for producing stable silica sols, the steps comprising preparing a substantially salt-free sol of active silica, the sol having a specific conductance, as measured at 2% S102 and 28 C., of not more than 17 10- mho/cm., by effecting contact, successively, between a sodium silicate solution and. a cation-exchanger in the hydrogen form and an anion-exchanger in the basic form, adding a suitable amount of lithium hydroxide to adjust the pH to between 6 and 9, heating the adjusted sol above 60 C. to form a heel, adding to the heel an additional quantity of a substantially salt-free sol of active silica while maintaining the pH in the range from 7.5 to 10.7, and the temperature at the boiling point of the sol, whereby water is evaporated, the additional quantity of sol of active silica being added at such a rate, relative to the rate of evaporation, that the volume of the combined sols remains substantially constant, and the amount of silica added being at least equal to that originally present, whereby build-up of the added silica upon that present in the heel is effected to give a sol product stable against gelation, total alkali additions being adjusted so that the product has an SiOztMzO ratio of 130:1 to 500:1, and concentrating the silica sol obtained to a silica concentration of not more than 45% by weight of SiOz.

10. In a process for producing stable silica sols, the steps comprising preparing a substantially salt-free sol of active silica, the sol having a specific conductance, as measured at 2% SiOz and 28 C., of not more than 1'7 10 mho/cm., efiecting contact, successively, between a sodium silicate solution and. a cation-exchanger in the hydrogen form and an anion-exchanger in the basic form, adding a suitable amount of sodium hydroxide to adjust the SiOztNazO mol ratio of the sol to from 130:1 to 300:1, heating the adjusted sol above 60 C. to form a heel, simultaneously but separately adding to the heel an additional quantity of a substantially saltfree sol of active silica and a quantity of sodium hydroxide sufiicient to maintain the pH in the range from 7.5 to 10.7 and the siozzNazO mol ratio from 130:1 to 300:1, and the same as in the heel, and maintaining the temperature at the boiling point of the sol, whereby water is evaporated, the additional quantity of sol of active silica being added at such a rate, relative to the rate 'of'evaporation, that the volume of the combined sols remains substantially constant, and the amount of silica added being at least equal to that originally present, whereby buildup of the added silica upon that present in the heel is effected to give a sol product stable against gelation, and concentrating the silica sol obtained to a silica concentration of not more than 45% by weight of 5102.

11. In a process for producing stable silica sols, the steps comprising preparing a substantially salt-free sol of active silica, the sol having a specific conductance, as measured at 2% S102 and 28 C., of not more than 1'7 10 mho/cm., by effecting contact, successively, between a sodium silicate solution and cation-exchanger in the hydrogen form and an anion-exchanger in the basic form, adding a suitable amount of sodium hydroxide to adjust the pH to between 6 and 9, heating the adjusted sol above 60 C. to form a heel, simultaneously but separately adding to the heel an additional quantity of a substantially salt-free sol of active silica and a quantity of sodium hydroxide sufficient to maintain the pH at a predetermined value within the range from 7.5 to 10.7 and the SiOz:Na2O mol ratio in the range from :1 to 500:1, and maintaining the temperature at the boiling point of the sol, whereby water is evaporated, the additional quantity of sol of active silica being added at such a rate, relative to the rate of evaporation, that the volume of the combined sols remains substantially constant, and the amount of silica added being at least equal to that originally present, whereby build-up of the added silica upon that present in the heel is effected to give a sol product stable against gelation, and concentrating the silica sol obtained to a silica concentration of not more than 45% by weight of S102.

12. In a process for producing stable silica sols, the steps comprising preparing a substantially salt-free sol of active silica, the sol having a specific conductance, as measured at 2% S102 and 28 C., of not more than 17 10 mho/cm., by effecting contact, successively, between a sodium silicate solution and a cationexchanger in the hydrogen form and an anionexchanger in the basic form, adding a suitable amount of sodium hydroxide to adjust the pH to between 6 and 9, heating the adjusted sol above 60 C. to form a heel, adding to the heel a quantity of sodium hydroxide sufiicient to give an ultimate product having an SiOmNazO mol ratio in the range from 30021 to 500:1 with a pre-determined amount of additional silica added as build-up, this amount of alkali being sufiicient also to maintain the pH in the range from 7.5 to 10.7, adding to the heel containing the sodium hydroxide an additional quantity of substantially salt-free sol of active silica while maintaining the temperature at the boiling point of the sol, whereby water is evaporated, the additional quantity of sol of active silica being added at such a rate, relative to the rate of evaporation, that the volume of the combined sols remains substantially constant, and the amount of silica added being at least equal to that originally present, whereby build-up of the added silica upon that present in the heel is effected to give a sol product stable against gelation, and concentrating the silica sol obtained to a silica concentration of not more than 45% by weight of S102.

13. In a process for producing stable silica 15 'so1s,-' :the :steps comprising .e preparing & an active silica 501. having aspecificiconductance, ,as-tmeasured at 2% "S102 and 28 0., .ofunot morethan 1'7 ,10- imho/cm zandan activity such that the silica, atzpH -12:and:0.02% SiOz, depo1ymerizes atzleast 90% to the monomer'in:not.:morethan 100 minutes, the active silicarsolv-being prepared by:jeffectingtcontact, successively, between :a; sodium silicate solution andnazcation-eexchanger in the. hydrogen .form .andn'an anion: exchanger in thefloasic form, adding axsuitableamount vof .sodium hydroxide to adjust :theSiOmNazOmoLratio of the S01 1120 from 130.: 1700 300: 1,- boiling the "adjusted soliunder atmospheric LDI'CSSUIIG to :form =a-:hee1,.:adding to the heel:anadditional wquantity of theroriginaliactive silicazsol, while'smaintainingtsthe pHfiin' /the rangezfrom37.5.;t0x10flzafld the :temp'erature i atstheivboilinggipointiof azthe. s01, whereby water .:-is evaporated, the additional quantity. zofia'ctive. silica: s01 :being .addedx'at such .a rate, Lrelative-tmthe :rate. of :evaporation, that .theviwolumec'of the combined: sols remains :substantially =;:c0nstant,:-'and the amount of silica added being at 1east-3 to :15 -1',imes-:.a's-v :much as thattoriginally present, whereby buildaup of the 'ad'dedi silica .up'onlthattpresent the heel :is effected toe-giveca solsproduct stablezagainstgela- .tion; and concentrating the: sol to; from to szby: weightofesilica.

.2 JOSEPHT. M.I RULE.

No references cited. 

1. IN A PROCESS FOR PRODUCING STABLE SILICA SOLS, THE STEPS COMPRISING PREPARING A SUBSTANTIALLY SALT-FREE SOL OF ACTIVE SILICA, THE SOL HAVING A SPECIFIC CONDUCTANCE, AS MEASURED AT 2% SIO2 AND 28* C., OF NOT MORE THAN 17X10-5 MHO/CM., ADDING A SUITABLE AMOUNT OF A BASE HAVING A MONOVALENT CATION TO ADJUST THE PH TO BETWEEN 6 AND 9, HEATING THE ADJUSTED SOL ABOVE 60* C. TO FORM A HEEL, ADDING TO THE HEEL AN ADDITIONAL QUANTITY OF A SUBSTANTIALLY SALT-FREE SOL OF ACTIVE SILICA WHILE MAINTAINING THE PH IN THE RANGE FROM 7.5 TO 10.7, THE AMOUNT OF SILICA ADDED BEING AT LEAST EQUAL TO THAT ORIGINALLY PRESENT, AND HEATING THE COMBINED SOLS ABOVE 60* C., WHEREBY BUILD-UP OF THE ADDED SILICA UPON THAT PRESENT IN THE HEEL IS EFFECTED TO GIVE A SOL PRODUCT STABLE AGAINST GELATION, TOTAL ALKALI ADDITIONS BEING ADJUSTED SO THAT THE PRODUCT HAS AN SIO2:M2O RATIO OF 130:1 TO 500:1, AND CONCENTRATING THE SILICA SOL OBTAINED TO A SILICA CONCENTRATION OF NOT MORE THAN 45% BY WEIGHT OF SIO2. 